The web team for the Lord of the Rings Online has published a guide on skinning the custom user interface. The write-up details step by step how to create a custom look to your in-game interface. A snippet is below:

In The Lord of the Rings Online^TM: Shadows of Angmar^TM (LOTRO), players can customize certain elements of the game's user interface (UI), also known as "skinning." While we don't allow you to do a full-scale UI replacement, we do give you the ability to replace selected visual elements of the UI. (A full list of these elements and their sizes is provided at the end of this overview.)

Click here for the guide.